In the metal sheet used for automobiles, building materials, home appliances and electric devices, a film comprising a chromium oxide is formed on the metal sheet surface to impart excellent corrosion resistance owing to the self-repairing function of the chromium oxide and thereby enhance the design property and corrosion resistance. Also, at the coating, a chromate treatment is applied as an undercoating treatment so as to enhance the corrosion resistance. However, from the standpoint of protecting the global environment, it is recently required to prevent the elution of chromate oxide. Consequently, a chromium-free surface film for the metal sheet or an undercoating treatment method not using a chromate treatment is being demanded.
In order to satisfy these requirements, for example, a resin chromate film obtained by compounding an organic resin and a chromate has been proposed in Japanese Unexamined Patent Publication (Kokai) No. 5-230666. However, this technique has a problem that the elution of chromium oxide can be decreased but cannot be completely prevented.
On the other hand, a treatment technique not using a chromate has been also developed. For example, Japanese Unexamined Patent Publication (Kokai) No. 11-29724 discloses a method of covering the metal sheet surface with a film comprising an aqueous resin having incorporated therein a thiocarbonyl group-containing compound, a phosphate ion and a water-dispersible silica. However, this technique has a problem that although the corrosion resistance is improved, the adhesion of the coating material is insufficient in uses where severe working is applied.
Also, Japanese Unexamined Patent Publication (Kokai) No. 8-73775 discloses an acidic surface treating agent containing two kinds of silane coupling agents. This technique has a problem that although good adhesion of the coating material can be obtained, the corrosion resistance is not satisfied.
Under these circumstances, the present invention provides a chemical conversion-treated metal sheet excellent in the corrosion resistance and coating adhesion, free from elution of chromium oxide and small in the environmental load.